1. Field of the Invention
The present invention is directed generally to monitoring operating parameters in a steam turbine generator and, more specifically, to the monitoring of turbine blade vibration.
2. Description of the Prior Art
Turbine blades, because of their complex design, can suffer from vibration at frequencies which correspond to natural frequencies of the blades called modes. Each mode is associated with a different type of vibration such as along the rotational axis of the turbine, perpendicular to the rotational axis of the turbine, etc. To prevent excessive vibration of the blade about its normal position, normal design practice dictates that the blades be constructed such that those modes are located between harmonics of the operating frequency of the steam turbine. However, manufacturing tolerances, changes in blade attachment to the rotor, changes in blade geometry due to erosion and changes in the operating frequency of the turbine, among other factors, cause mode frequencies to approach harmonics of the operating frequency. Additionally, damaging nonsynchronous vibration may also occur. Typically, nonsynchronous vibration in a steam turbine may occur as a result of buffeting wherein a low steam flow and a high back pressure cause the random excitation of the turbine blades or as a result of turbine rotor torsional stresses.
The approach of the modes to the harmonics of the operating frequency may result in physical damage to the steam turbine. When the amplitude of the vibration exceeds a certain level, objectionable stresses are set up in the blade. If the condition is not detected and remedied, the blade may eventually fracture resulting in an extremely costly forced outage of the machinery. Thus, a method for detecting that vibration is necessary to prevent such damage.
The prior art method for detecting turbine blade vibration is to attach strain gages to the turbine blades. The strain gages measure the vibration of the turbine blades to which they are attached and that information is communicated to analyzing equipment outside the machine by means of miniature transmitters affixed to the machine's rotating shaft at various locations.
That prior art method suffers from three significant drawbacks. First, the strain gage has a very short life due to erosion caused by steam passing through the turbine blades. Second, each blade requires a strain gage if all blades in a row are to be monitored. That represents a significant expense. Additionally, only a limited number of transmitters and, therefore, strain gages can be accommodated inside the machine. Third, the complexity of continuously and reliably supplying power to the strain gage and transmitting the signal reliably from the rotating rotor disk to stationary electronics creates severe difficulties.
To obviate those problems, apparatus exist for detecting turbine blade vibration which utilize permanently installed, non-contacting proximity sensors. One such apparatus is disclosed in U.S. Pat. No. 4,573,358 wherein a plurality of sensors spaced about the periphery of the blade row detects vibration of operator selected blades. Typically, the type of noncontacting sensor used is a magnetic sensor which induces eddy currents in the blade tip. Those eddy currents create a magnetic field which is sensed by the sensor. Thus, the apparatus is dependent upon the sensor's ability to induce eddy currents in the blade. Such an apparatus cannot function in a turbine which has blades made of non-magnetic materials, materials in which it is extremely difficult to induce eddy currents, or materials which lose their magnetic properties at the temperatures and pressures at which the turbine operates. The apparatus also requires a clear line of sight between the sensor and the blade tip, and fairly close proximity between the sensor and blade tip for the sensor to be effective.
Thus, there is a need for a long-lived monitor that can function on turbines having blades made of magnetic material as well as on turbines having blades made of non-magnetic material. The need also exists for a vibration monitor that can operate with a sensor that need not be in close proximity to the movement being sensed and does not require a clear line of sight.